Known radio telephones have variable-gain power amplifiers for amplifying modulated radio-frequency ("RF") signals prior to transmission. The gains of the power amplifiers are typically controlled by automatic gain control ("AGC") circuits. Conventional AGC circuits operate dynamically to maintain the amplifier outputs within defined power output tolerances or ranges, and usually at or near nominal values (e.g., intermediate points) within those ranges. Various factors, including changes in ambient temperature and in power supply (e.g., battery) power level, can cause the amplifier output to vary from the targeted output power level. It is the job of the AGC circuits to correct for such variations.
For cellular telephony, generally recognized standards specify the nominal value and a range or tolerance for the amplifier output power. For instance, the Electronic Industries Association/ Telecommunications Industry Association Standard "Mobile Station-Land Station Compatibility Specification", EIA/TIA-553, September, 1989, Section 2.1.2.2. specifies that the power level must be maintained within a 6 dB range from +2 dB to -4 dB of its likewise-specified nominal level over the ambient temperature range of -30 degrees Celsius to +60 degrees Celsius and over the supply voltage range of .+-.10 percent from the nominal value. Thus, according to that specification, the amplifier output power must be controlled by the AGC circuit so as to fall within a 6 dB range about a nominal power level.
Conventional AGC circuits in such telephones typically target the nominal power level regardless, e.g., of the distance of the station that is to receive the transmitted signal or of other conditions which would otherwise suggest the transmission of stronger or weaker signals from the telephone.
In developing transmission signals of that power level, the transmitter's power amplifier itself consumes most of the dc power required by the radio telephone during audio or voice communication (called "talktime"). As radio telephones become increasingly self-powered, e.g., through the use of on-board, rechargeable storage batteries, its power needs have become an increasingly significant design consideration.
This comes into focus when one considers that, typically, the greater the power requirements of the power amplifier, the shorter the talktime on a single charge of the batteries in the telephone. If steps could be taken to reduce the power requirements by, e.g., reducing the output power of the transmitter amplifier, performance of the telephone could be improved.